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Micro Magnum, Team Magnum, NSW
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Nick
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Joined: 16 Jun 2004
Posts: 11802
Location: Sydney, NSW


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I try to thread screw holes at least 2 times as deep as the width of the screw, deeper if possible. If you are talking about threading to the bottom of a blind hole, you need to use two taps. The first tapered tap gets the thread started and part way down the hole and then the the bottoming tap will cut threads almost the bottom of the hole. Its difficult to cut threads right to the bottom of a hole and you should always make the hole a little deeper than the screw.
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Post Sun Apr 23, 2017 8:08 am 
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maddox



Joined: 21 Dec 2006
Posts: 786
Location: Belgium


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Rule of tumb.

Thread depth is always a multiple of the diameter. Material is the other parameter.

Considering bolt material 8.8 or better.


Steel. Dx1
Bronze/Brass Dx1.5
Titanium Dx1
Ali (good quality) Dx2
Ali cheese grade Dx4
Magnesium Dx2.5

Most plastics ain't very good thread retainers, but for polycarbonate Dx4 is a good idea.

Post Sun Apr 23, 2017 9:53 am 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Thank you guys for your help.

Still I have a question, you mentioned I should make the hole deeper than the thread, and I wonder if 2mm will be enough.

Post Sun Apr 23, 2017 4:43 pm 
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maddox



Joined: 21 Dec 2006
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Location: Belgium


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Depends on thread. For an M3 2mm is good, for an M8 it's not enough enough.

2 things that influence the "extra depth" needed.

Type of tap used. There are special taps for blind holes. Those can tap to the end - 3 threads.

And that brings us to the pitch. A normal M3 has a pitch of 0.5mm. An M8 has a pitch of 1.25mm.

If you use normal taps, you'll have to clean out the hole very regulary, and the first 10 threads ain't fully formed.

Post Sun Apr 23, 2017 5:16 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Ah I see, thanks for the info.

Post Sun Apr 23, 2017 5:35 pm 
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Nick
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Joined: 16 Jun 2004
Posts: 11802
Location: Sydney, NSW


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Most bottoming taps have 2 to 3 partial threads at their tip, so you want to make the holes 3 times the thread pitch plus a safety margin. IE: for an M6 x 1 leave 3mm plus maybe one extra mm for safety. If you are doing the tapping yourself, you can leave less but as you are outsourcing, its best to play it safe.
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Post Sun Apr 23, 2017 5:39 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Yeah you're right.

I just did some searching online, and some say that a fine thread may be a bit stronger (and they might also save some extra depth). However I couldn't really find anyone selling fine thread screws on Ebay. Why is that?

EDIT: NVM I think I should go with the coarse ones for compatibility and repairing reasons since they seem to be more prevalent.

Post Sun Apr 23, 2017 6:19 pm 
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Nick
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Joined: 16 Jun 2004
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Location: Sydney, NSW


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I don't know where all this fine threads are stronger BS comes from - in softer materials like aluminium, coarse threads are definitely stronger.
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Post Sun Apr 23, 2017 7:52 pm 
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MoonSet416



Joined: 25 Sep 2016
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Location: Sydney


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OK I see... Thanks Nick.

Post Sun Apr 23, 2017 8:17 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Did a few design checks and made some tweaks to the chassis...
Now I think Contraption IIC will go to the next stage.
I also found a few things that I am not so sure about, so I would like to ask those questions here:

1.About the material and also the heat treatment of the disc, what are some common choices besides S7 (which might be tricky to heat treat).

2.Any recommendations for machine shops? (since I will be outsourcing the parts) Also, do I need to buy the material and send it to them or do I just choose from the materials in their stock?

Post Wed Apr 26, 2017 12:24 am 
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chunkulator



Joined: 27 Jul 2016
Posts: 219


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I was asking similar questions a few months ago. Check out this thread: http://www.robowars.org/forum/viewtopic.php?t=2263

The consensus was that S7 is a bad idea. It's been tried and has failed before due to the difficulties of finding a local supplier who can heat treat it right. What you want is laser or water jet cut Hardox 400, 450 or 500 or Bisalloy. Many of the laser cutters hold these materials in stock.

Action Laser is Perth was recommended to me for cutting out the weapon, but I haven't gone ahead and sent them drawings yet because I've been concentrating on sportsmen for Vivid.

Post Wed Apr 26, 2017 6:44 am 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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I see... Thanks for the info.

Post Wed Apr 26, 2017 6:52 pm 
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Nick
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Joined: 16 Jun 2004
Posts: 11802
Location: Sydney, NSW


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While I like S7, the harder grades of Bisalloy and Hardox will stay sharp almost as long and are much less likely to fracture. When you include the cost of heat treating, they will also be cheaper than S7. The only reason to choose S7 is if the part has complicated machining and threading, which is much easier in annealed S7 than in pre-hardened steels.

I have nearly identical beaters in S7 and Bisalloy 450 for Mr Mangle; two early designs in S7 cracked and the current S7 beater is staying sharper than the Bisalloy beater but is also getting more chips in the cutting edges.
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Post Wed Apr 26, 2017 7:05 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Just had a look at some of the spec sheets of hardox and bisalloy, and I have to say I'm a bit confused. It seems to me that as the grade number of an alloy goes up (say from hardox 450 to 500), it gains hardness and yield strength but the impact energy will decrease. So I wonder if it is right to say that higher grade plates are in general harder and more brittle?

I also heard that laser cutting may have some minimal radius related to the plate thickness that it can cut... Is that true? (I previously thought that laser would cut tight corners, so I had some small radiuses in my ali plates, like 3mm chamfers)

Post Wed Apr 26, 2017 7:53 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Yeah I think hardox or other wear plates will work better for me, they can probably save me some time and money.

Post Wed Apr 26, 2017 7:55 pm 
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